Chlorine acts as both a sanitizer and oxidizer in pools. Sanitation kills bacteria and microorganisms, while oxidation chemically breaks down and removes contaminants from the water. Oxidation, also called shocking, is needed routinely to reduce the contaminant load on the sanitizer and preserve water clarity. It breaks down remnants of dead bacteria, plant materials, bather-introduced oils and waste, and materials from rain or runoff. The oxidation-reduction process involves atoms gaining or losing electrons to reach their ideal oxidation state, allowing contaminants to be more easily removed. Oxidation-reduction potential (ORP) measures this tendency on a scale of -2000mV to +2000mV, with values

1. BIO301: OXIDATION
Pool & Spa University

2. QUICK REVIEW
➤ Chlorine is effective as both a sanitizer and an oxidizer. However, these two
functions are distinct.
➤ Sanitation is defined as the process by which organisms are destroyed. In pools,
this generally refers to killing bacteria and other microorganisms.
➤ Oxidation is defined as the process by which contaminants are chemically changed
(broken down) and removed from the system.
➤ All pools require continuous sanitation and routine oxidation.
Regardless of sanitizer choice, there must always be sanitizer present to
prevent the spread of waterborne illnesses. Oxidation should be performed
on a regular and as-needed basis to reduce contaminant load in the system.
➤ Only products registered with the EPA are appropriate for use as
sanitizers and oxidizers. BioGuard® products are EPA-registered for use
in swimming pools and spas.

3. OXIDATION
➤ As previously stated, oxidation is the process by which the structure of
contaminants is changed, making them easier to remove from the system.
➤ Generally, this is referred to as “shocking” the system.
➤ Oxidation is designed to remove a variety of materials in pool water:
➤ Remnants of dead bacteria from sanitation
➤ Plant material, including dead algae
➤ Bather-introduced materials (oils, lotions, sweat, waste)
➤ Contaminants brought in from rainwater or run-off
➤ Routine oxidation is required to reduce the level of contaminants in the water,
thereby reducing strain on the primary sanitizer and preserving water clarity.
➤ When we look at how chlorine (HOCl specifically) oxidizes material, we look
at oxidation-reduction reactions.

4. OXIDATION-REDUCTION REACTIONS
➤ Oxidation-reduction reactions refer to the exchange of electrons and require two items: the oxidant
and the reductant.
➤ The oxidant will lose electrons and will be reduced.
➤ The reductant will gain electrons and will be oxidized.
➤ How well two materials will react depends on an atom’s oxidation number and ideal oxidation
number (ideal oxidation state). This allows you to see how easy/difficult it will be to remove a
contaminant. Atoms that are not in their ideal oxidation states are reactive and will respond readily in
order to reach their ideal oxidation number.
➤ Example: The chlorine in hypochlorous acid has an oxidation number of +1 and wants to be at -1;
therefore, it will lose electrons and be reduced. Chlorine is an oxidizer.
➤ Example: The nitrogen in ammonia (NH3) has an oxidation number of +3 and wants to be at +5;
therefore, it will gain electrons and be oxidized. Nitrogen is a reductant.
➤ Example: The phosphorus in orthophosphate (PO4
-3) has an oxidation number of +5 and wants to
be at +5. It already is at its ideal oxidation state; it will not react with reductants or oxidizers.
➤ While knowing the practical science behind how oxidation works is useful, customers 99% of the time
don’t know and likely will not care. That said, it’s useful to understand how it works on the atomic
level to recognize where reactions will occur (ammonia) and where they will not (phosphates).

5. MEASURING OXIDATION-REDUCTION IN POOL WATER
➤ When measuring oxidation-reduction reactions in pool water,
we refer to the oxidation-reduction potential (ORP) of the
water.
Reduction potential (also known as redox potential, oxidation /
reduction potential, ORP, pE, or ε) is a measure of the tendency of a
chemical species to acquire electrons and thereby be reduced.
Reduction potential is measured in volts (V), or millivolts (mV).
Wikipedia
➤ The ORP scale runs from -2000mV to +2000mV; values
under 0 indicate a substance likely to be oxidized, while
values above 0 indicate a substance likely to be reduced.

6. USING ORP IN POOLS
➤ For pools, a value of greater than +650-700mV is desired to
ensure proper disinfection of the water.
➤ The World Health Organization (WHO) recommends an
ORP reading of +750mV or greater.
➤ ORP readings can be made with large chemical controllers
which dispense chlorine and acid into a pool as needed based
on the readings obtained from a series of probes; readings can
also be taken from handheld ORP meters.
➤ It’s important to remember that ORP is not generally a
method of measurement required by state health
departments.

7. CONGRATULATIONS!
You have completed the activity!